GSTL maintains two types of California bearing ratio (CBR) test frames:
1.1 This test method covers the determination of the California Bearing Ratio (CBR) of soil tested in place by comparing the penetration load of the soil to that of a standard material. This test method covers the evaluation of the relative quality of sub-grade soils, but is applicable to sub-base and some base-course materials.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Subgrade Strength
The strength of the subgrade (soil below the pavement) is asssesed using a test known as the California Bearing Ratio test. This was developed in California in the 1930's and makes no attempt to determine any of the standard soil properties such as density. It is merely a value and it is integral to the process of road design. Nearly all design charts for the road foundations are based on the CBR value for the subgrade.
3. California Bearing Ratio
The California Bearing Ratio (CBR) is a measure of the supporting value of the subgrade. It is not unique and other tests such as the R-Value test and the Triaxial are used occasionally. It is however by far the most commonly used in Pavement Design. The CBR test should be used with soil at the calculated equilibrium moisture content .
BS 1377 : Part 9 : 1990
18.104.22.168 Reaction Load
The provision of the reaction load, incorporating the frame on which to attach the jack, may be in any convenient form such that it acts as a stable dead weight with its normal supports placed sufficiently far from the jack not to influence the results of the soil being tested.
The plunger is then seated into the soil using a force of 50N for an expected CBR below 30% or 250N for greater than 30%.
The plunger is then penetrated into the soil at a constant rate of 1mm/min and the forces recorded at penetration intervals of 0.25mm. The total penetration should not exceed 7.5mm.
These results are then compared to a standard curve for a value of 100% CBR. The forces on the standard curve are 13.2kN at 2.5mm penetration and 20.0kN at 5.0mm penetration.
The CBR is then a simple ratio of the corresponding values and where a difference between the value at 2.5mm and 5mm occurs, the higher value is taken. Annular weights are sometimes used to represent a surcharge.
The first factor affecting the performance of the sub-grade is the moisture content. Unfortunately this is normally extremely variable as water can come from many sources such as rainfall, capillary action, seasonal movement of the water table and ingress.
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